REACTIVITY OF 1-IODOADAMANTANE WITH CARBANIONS BY THE S(RN)1 MECHANISM

The photostimulated reaction of 1-iodoadamantane (1) with carbanions i n DMSO has been studied. Acetone enolate ion (2a) reacted with 1 to gi ve the substitution product (4a, 20%) and adamantane (3, 17%); acetoph enone enolate ion (2b) gave the substitution product (4b, 65%) in the presence of 18-crown-6. In the reaction with propiophenone enolate ion (2c), the yield was lower (27%). Anthrone anion (5) gave good yields (75%) of the substitution product 6. The anion of nitromethane (7) did not react with 1 under photostimulation: but in the presence of 2a or b, 87% of 1-adamantylnitromethane (8) was obtained (entrainment react ions). Low amounts of substitution products were found with the fluore ne anion and no substitution was observed with other carbanions. It is suggested that these reactions occurred by the S(RN)1 mechanism. By c ompetition experiments, the following reactivity order toward 1-adaman tyl radicals was determined: 2a (1.0) < 2b (11) < 7 (32) < 5 (80). The reactivity observed is explained on the basis of the pK(A) of the con jugate acid of the anions and on the changes in pi energy (HOMO carban ion - SOMO radical anion intermediate) that takes place in the couplin g reaction. By competition experiments of 1 and Phl toward 2b, a k(1): K-Phl ratio of ca. 16 was found while 1-bromoadamantane and PhBr gave a k(1-BrAd):k(PhBr) ratio of ca. 0.13. This difference can be attribut ed to the capacity of phenyl radicals to abstract iodine atoms from 1 giving Phl and 1-adamantyl radicals, which ultimately results in an ap parent greater reactivity of 1 vs. Phl.